1. Field of the Invention
This invention relates to a high-voltage circuit for a post focusing type color picture tube or more in particular to a high-voltage circuit the output voltage of which undergoes little variations against load variations.
2. Description of the Prior Art
In conventional color picture tubes of shadow mask type, the amount of electron beams transmissible through the shadow mask is limited and therefore, if the brightness of the phosphor screen is to be increased, the velocity of electrons constituting the beams must be increased, that is to say, the high voltage applied to the anode must be made higher. Increasing the voltage applied to the anode makes it necessary to increase the deflection power, which in turn requires a higher power of the deflection output circuit.
The color picture tube of post focusing type has been developed to increase the brightness without increasing the anode voltage or deflection power required. This post focusing type color picture tube is such that the phosphor screen and the shadow mask are electrically insulated from each other and impressed with different voltages. The color picture tube of this type is also characterized by an anode provided at the funnel section. Thereby, the rate of transmission of electron beams through the shadow mask is increased, thus making it possible to obtain a high brightness without increasing the high voltage applied to the phosphor screen.
In the color picture tube of this type, the relations Em .apprxeq. Ef and Eb &gt; Em are established, where Em is the voltage applied to the shadow mask, Ef the voltage applied to the anode of the funnel section, and Eb the voltage applied to the phosphor screen. Eb being substantially at the same level as the anode voltage of the conventional color picture of shadow mask type. Since the voltage of the funnel section anode contributing to deflection is lower than the anode voltage of the conventional shadow mask type color picture tube, a smaller deflection power than conventionally needed suffices. It is necessary, however, to maintain both Eb/Em and Eb/Ef constant for the reason that if the brightness is changed by a brightness adjustment process or if Eb/Em or Eb/Ef changes for lack of uniformity of characteristics of component elements, the post focusing cannot be effected as required, resulting in the deterioration of color purity. The variations allowable for Eb/Em and Eb/Ef are very slight, making design and adjustment of the high voltage circuit of the color picture tube very much complicated.